It is desired that a speed of a motor used for a VTR be constant. When the speed is varied causing such as speed variation, speed ripple, or torque ripple, a picture of the VTR is disturbed and a reliability thereof is remarkably decreased.
Conventionally, an apparatus in this field chiefly uses a DC motor. Recently, a brushless motor has been used for the apparatus since a speed of the motor is able to be changed freely and simply.
Since the brushless motor has no mechanical brushes, it does not suffer from abrasion of brushes and commutators or trouble caused by abrasion powder of the brushes and commutators of the motor. On the contrary, magnetic flux linkage numbers at conducting coils of 120 degree conducting type brushless motor are changed corresponding to rotor positions causing the torque ripple so that the speed variation is produced.
Suppose that a magnetic flux linkage number of a driving phase is K(.theta.), wherein .theta. is a position of the movable equipment.
A generated torque is represented by K(.theta.) I, wherein I is a current value passing through the coils.
When a conducting current is constant, the torque ripple is generated proportional to K(.theta.) as well as the magnetic flux linkage numbers.
On the other hand, an example, whose conducting current to the coils is 1/K(.theta.) corresponding to the rotational positions of the rotor, is disclosed in FIGS. 2 and 3 of Japanese Patent Laid-Open No. 55-79694 published on June 16, 1980 entitled "Motor Control Circuit".
Such a conventional motor control circuit discloses that an information of 1/K(.theta.) is stored to a ROM (Read Only Memory) previously corresponding to a rotational position of a rotor, the information of 1/K(.theta.) is obtained from the ROM corresponding to the rotational position, and a current instruction signal is compensated to 1/K(.theta.).multidot.I.sub..tau. so that a constant torque is obtained, wherein I.sub..tau. is a current instruction value.
Moreover, a brushless motor having a core generates a large speed variation caused by cogging torque.
Since the magnetic flux linkage number K(.theta.) is varied based on states of magnetized distributions of permanent magnets, and arrangements of the wounded coils of the conventional motor control circuit disclosed in Japanese Patent Laid-Open No. 55-79694, it is difficult to detect the compensation signal 1/K(.theta.) correctly in each motor.
Although the cogging torque is generated at the motor which has core, a magnitude and a phase of the cogging torque are varied remarkably depending on unbalanced magnetizing states and assembling errors of the core. It is impossible to compensate the torque ripple and speed variation perfectly based on the data memorized at the ROM.
A learning control method for suppressing the torque ripple is disclosed, for instance, in FIGS. 3 and 4 of Japanese Patent Laid-Open No. 61-173690 published on Aug. 5, 1986 entitled "Speed Control Apparatus of Motor" in which integration terms used for speed control are prepared independently corresponding to the rotational positions and these integration terms are changed over sequentially corresponding to the rotor positions.
However, in the method disclosed in Japanese Patent Laid-Open No. 61-173690, many Random Access Memory (RAMs) have to be prepared corresponding to the pulse numbers generated from a Frequency Generator (FG) so that the method has a drawback that a control apparatus becomes complex and high in cost.